FIG. 13 and FIG. 14 show conventional, typical optical switch structures each of which uses a multimode slab waveguide. These structures are disclosed by Patent Document 1 titled “Optical Switch” and Patent Document 2 titled “Multimode Interference Waveguide Type Optical Switch,” respectively.
In the structure of FIG. 13, two parallel electrodes are arranged on a multimode interference device, and a switching operation is performed to modulate the refractive index of a waveguide disposed under the electrodes by applying voltage or by injecting current to the electrodes. Reference sign 201 designates a multimode interference waveguide, which consists of two parts 202a and 202b. Reference sign 203 designates an input waveguide. Reference sign 204 designates an output waveguide. Reference signs 205a and 205b designate electrodes. In the conventional technique, the refractive index of a part to which a voltage has been applied is increased, and, when a voltage is applied to both electrodes, an optical signal is confined in a part disposed under the electrodes, and a switch performs a bar operation (i.e., connection between input and output waveguides that face each other). In a state in which no voltage is applied, the switch acts as a multimode interference waveguide that makes a cross connection. The problem of this structure is that it is necessary to vary the refractive index all over the area in which light is guided and that the widened area of the electrode causes an increase of electric power necessary for switching and causes an increase in switching time resulting from an increase in capacitance. Additionally, although this conventional technique uses the method of increasing the refractive index of a part disposed under the electrode, the plasma effect by injecting current into the semiconductor and the electro-optic effect of material, such as PLZT (lanthanum-doped lead zirconate titanate), cannot be used in the structure of this conventional technique, because the refractive index is decreased by injecting current or applying voltage.
In the structure of FIG. 14, two divided electrodes are provided at the center part of the multimode interference device, and switching is performed while changing an optically confined state by modulating the refractive index of the waveguide disposed under the electrodes. Herein, reference sign 206 designates a multimode interference waveguide, which consists of two parts 207a and 207b. Reference sign 208 designates an input waveguide, reference sign 209 designates an output waveguide, and reference signs 210a and 210b designate electrodes. In this prior art device, the refractive index of a part disposed under the electrodes of the multimode interference waveguide 206 is decreased by applying a voltage to the electrodes, and the multimode interference waveguide 206 is optically divided into two parts, i.e., into a multimode interference waveguide 207a and a multimode interference waveguide 207b. In other words, when no voltage is applied thereto, the switch acts as a multimode interference waveguide that makes a cross connection. On the other hand, when a voltage is applied thereto, the switch acts as two multimode interference waveguides that are connected together between an input and an output that face each other. In this structure, in a bar state, one side surface of the multimode interference waveguide is optically confined by a change in the refractive index resulting from the application of a voltage, and the other side surface thereof is optically confined by use of a difference in the refractive index between the waveguide side and the outside. Therefore, asymmetry becomes high with respect to a direction in which light is guided and propagated, and a tolerance of the amount of refractive-index variation becomes small. Therefore, it is difficult to improve the crosstalk and extinction ratio characteristics. Additionally, a change in the refractive index necessary for switching is determined by the propagation angle of the highest mode excited at the end of an input waveguide. A change in voltage necessary for switching is substantially proportional to a change in the refractive index needed. In other words, a problem resides in the fact that a switching voltage becomes high. Additionally, no consideration is given to the fact that the area of the electrode is large, and hence electric power necessary for switching is increased, and an increase in switching time resulting from an increase in capacitance is caused. Additionally, design considering a finite electrode width is not formed.    [Patent Document 1] Japanese Published Unexamined Patent Application No. H7-110498    [Patent Document 2] Japanese Published Unexamined Patent Application No. 2001-183710